Investments - Loans by U.S. Region (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 3,732.1	$ 3,514.1
Loans by region percentage of total loans	100.00%	100.00%
Pacific
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 891.5	$ 802.6
Loans by region percentage of total loans	23.90%	22.80%
South Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 846.4	$ 746.5
Loans by region percentage of total loans	22.70%	21.20%
West South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 445.0	$ 448.4
Loans by region percentage of total loans	11.90%	12.80%
Middle Atlantic
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 524.1	$ 505.8
Loans by region percentage of total loans	14.00%	14.40%
East North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 361.8	$ 355.3
Loans by region percentage of total loans	9.70%	10.10%
Mountain
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 288.4	$ 274.0
Loans by region percentage of total loans	7.70%	7.80%
West North Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 214.7	$ 219.6
Loans by region percentage of total loans	5.80%	6.30%
East South Central
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 76.0	$ 87.1
Loans by region percentage of total loans	2.00%	2.50%
New England
Open Option Contracts Written [Line Items]
Total commercial mortgage loans	$ 84.2	$ 74.8
Loans by region percentage of total loans	2.30%	2.10%